1. Field of the Invention
The present invention relates to a pole plate structure for a motor stator. More particularly, the present invention relates to a pole plate structure for a brushless d.c. motor stator that increases the rotating torque of the motor while allowing easy starting of the motor and reducing the cogging torque.
2. Description of the Related Art
A conventional cooling fan motor, as illustrated in FIGS. 1 and 2 of the drawings, discloses a stator for a brushless d.c. motor comprising an upper pole plate 91 and a lower pole plate 92 respectively mounted to upper and lower ends of a set of windings 93. Each pole plate 91, 92 comprises a plurality of vertically extending magnetic pole faces 94 for increasing a rotating torque of the rotor. In order to allow easy starting and avoid dead corners in rotation of the rotor, each magnetic pole face 94 includes a chamfered corner 95. However, since the permanent magnet of the rotor is comprised of alternatively disposed north poles and south poles and since the rotor is driven by the alternating magnetic fields generated by the stator winding, changes in the rotating torque or uneven rotating torque (the so-called xe2x80x9ccogging torquexe2x80x9d) occur during a transient change between the north pole and south pole of the permanent magnet of the rotor relative to the magnetic pole faces of the upper and lower pole plates 91 and 92. As illustrated in FIGS. 1 and 2, at least an edge 96 of the magnetic pole face 94 represents a rectilinear line that is perpendicular to a planar surface of the pole plates 91, 92, which generates a significant cogging torque during rotation of the rotor. Trembling of the rotor as a result of cogging torque is more obvious when the rotor rotates at low speed.
An object of the present invention is to provide a pole plate structure for a motor stator having a larger relative induction area between the rotor and the stator magnetic poles to thereby increase the rotating torque of the rotor. In addition, the pole plate structure allows easy starting of the rotor and reduces the cogging torque during rotation of the rotor.
A pole plate for a motor stator in accordance with the present invention comprises a central hole and a plurality of circumferentially regularly spaced sector edges. Each sector edge has a pole face extending in a plane perpendicular to a planar surface of the pole plate. The pole face includes a distal side that is opposite to the respective sector edge. A central line passing through a middle point of the respective sector edge and a middle point of the distal side of the respective pole face are not coincident with and at an angle with a radial line passing through a center of the central hole and the middle point of the respective sector edge.
Other objects, specific advantages, and novel features of the invention will become more apparent from the following detailed description and preferable embodiments when taken in conjunction with the accompanying drawings.